The present invention concerns liquid crystalline dielectrics for electro-optical indicator elements operating on the basis of the guest-host effect and containing one or more pleochroitic naphthoquinone dyestuffs.
In the case of electro-optical indicator elements with liquid crystalline dielectrics, it is known to produce the electro-optical effect of the indication by incorporation of dichroitic or pleochroitic dyestuffs as the so-called "guest phase" into a liquid crystalline matrix which is the so-called "host phase" (G. H. Heilmeier et al., Molecular Crystals and Liquid Crystals, Volume 8 (1969), pages 293-304). The dyestuff molecules of the guest phase are oriented by the embedding host phase in which they are dissolved or distributed corresponding to the applied electrical field. Because of their pleochroitic properties, they show different light absorption depending upon the orientation.
In contradistinction to normal monochroitic dyestuffs, the amount of light absorbed by pleochroitic dyestuffs depends upon the orientation of their molecules to the electric field vector of the incident light. By application of an electrical field to a thin layer of dielectric distributed in a cell, the nematic liquid crystals of the host phase and, together with these, the pleochroitic dyestuffs incorporated as guest phase are reoriented. This produces a change in light absorption. The practical use of this technique, referred to in the literature as the guest-host effect, is described e.g. in published Federal Republic of Germany Patent Specification No. 1,928,003 (U.S. Pat. No. 3,597,044), whose disclosures are incorporated by reference herein. As examples of pleochroitic or dichroitic dyestuffs, there are described indophenol blue, indigo derivatives, azo dyestuffs and the like. The contrasts thereby achievable are only sufficient for good readability when used with a polarization foil.
According to a recent development, which is described e.g. in published Federal Republic of Germany Patent Specification No. 2,410,557 (U.S. Pat. Nos. 3,833,287 and 3,837,730) polarizers can be omitted when to the dielectric composed of nematic liquid crystals with positive dielectric anisotropy (as host phase) and pleochroitic dyestuff incorporated therein (as guest phase), there is added a small amount, e.g. 0.1-15 wt.% of an optically-active material which brings about in the liquid crystal the formation of a screw-shaped structure (cholesterinic structure). Examples of such compositions, as well as the physical bases of the light absorption therein, are described in White et al, "New Absorptive Mode Reflective Liquid Crystal Display Device" (J. Appl. Physics, Volume 45 (1974), pages 4718-4723). Examples of the construction and operation of electro-optical display elements based on this effect are described in published Federal Republic of Germany Patent Specifications Nos. 2,639,675 (U.S. Pat. No. 4,213,676) and 2,658,568.
In the literature, the liquid crystal indicator elements with cholesterinically oriented host phase and dyestuff embedded therein as guest phase are frequently referred to as cholesteric guest-host displays. These CGH indicator elements have proved to be advantageous because, without polarization foils, they provide a good display contrast and, in addition, a greater brightness of the indication formed.
It has been shown that the selection of suitable dyestuffs for CGH display elements is very difficult. In the first place, the dichroitic ratio values of such systems with host phase and dyestuff embedded therein as guest phase must be sufficiently large in order to impart to the indicator cell a sufficient brightness and a sufficient contrast ratio. The achievable contrast thereby depends upon the degree of order S of the dyestuff in the liquid crystalline matrix. The degree of order can be compared by the relationship ##EQU1## wherein R is the angle between the molecular longitudinal axis of the dyestuff molecule and the optical axis of the liquid crystal; E.sub.II and E.sub.I are the values of the extinction of the indicator element when the measurements are carried out in parallel orientation (E.sub.II) or vertical orientation (E.sub.I) of the liquid crystal molecules.
A value which is easier to deal with in every day practice and which still permits a measure of the degree of order S and thus of the technical useability of a dyestuff-liquid crystal combination, is the so-called dichroitic ratio V, which represents the quotient of the above-defined extinction values: EQU V=E.sub.II /E.sub.I
In technically useable dyestuff-liquid crystal combinations, the value of B is 5 or above.
In practice, the degree of order of a dyestuff depends, in the first place, upon its chemical structure, as well as upon the nature of the liquid crystalline matrix. A number of examples is described by R. I. Cox in "Molecular Crystals and Liquid Crystals", Volume 55 (1979), pages 1-33".
In addition to the degree of order or the dichroitic ratio, other parameters are also important for the use of a dyestuff in CGH indicator elements. An absorption maximum of the dyestuff must lie in the visible range, i.e. between 400 and 700 nm. The dyestuff must be stable in the doped host phase up to a temperature of about 100.degree. C. against alternating voltages of up to 20 V, to radiation in the infra-red, visible and ultra-violet ranges and to the components of the liquid crystalline host phase. Furthermore, good solubility in the host phase and a high optical density are desirable.
However, the dyestuffs hitherto suggested for CGH indicator elements do not fulfill these requirements to a sufficient extent. In particular, the azo dyestuffs usually suggested as suitable for use in CGH indicator elements with regard to their dichroitic ratios and absorption maxima have proved to be insufficiently stable towards visible light or ultra-violet radiation (too low photochemical stability). In some cases, they are also chemically unstable. On the other hand, the photochemical and chemical stability of anthraquinone dyestuffs, which is generally higher in comparison with azo dyestuffs, is known. Proposals have been made in the literature for the use of anthraquinone dyestuffs for CGH indicator elements. However, most of the suggested anthraquinone dyestuffs have too small a dichroitic ratio V to be technically useable in CGH indicator elements.
In published British Patent Application No. 20 11 940 there are described substituted anthraquinone dyestuffs of the formula (A), ##STR3## wherein R.sub.1 is an aniline group, optionally substituted in the ring and attached via the nitrogen atom; R.sub.2 is H or OH; and R.sub.3 is H or, when R.sub.2 is H, also an optionally substituted aniline group. The dichroitic ratio of these anthraquinone dyestuffs, measured in commercially available cyanobiphenyl/cyanoterphenyl liquid crystal mixtures, lies between 3.8 and 8.4, and most values lie between 5 and 6. However, the solubility of the dyestuffs in most conventional liquid crystalline host phases is reduced due to the aniline group. In particular, in the liquid crystalline base materials of the phenylcyclohexane class today used to a large extent, the solubility of these dyestuffs of formula (A) is too low for practical requirements.